The ultraspiracle (Usp) gene of Drosophila melanogaster encodes a protein that is required throughout the development of flies (Oro et al., Develop. 115:449, 1992). It was first identified by the phenotype of mutants that died in the molting process between the first and second instar stages and thus had two sets of spiracles (Perrimon et al., Genetics 111:23, 1985). The behavior of these mutants suggested that Usp functions in the ecdysone (Ec) response pathway of metamorphoses and imaginal disk formation. It has subsequently been found that Usp also functions in female reproduction and eye morphogenesis, and may participate in Ec-dependent and Ec-independent processes in the fly. (Garen et al., Proc.Natl.Acad.Sci.USA 74:5099, 1977; Segraves, Rec.Prog.Horm.Res. 49:167, 1994).
Cloning the Usp gene revealed sequence homology between Usp and the vertebrate retinoic acid X receptors (RXRs) (80% identity in the DNA binding domain and 49% in the hormone binding domain) (Henrich et al., Nuc.Acids.Res. 18:4143, 1990; Shea et al., Genes Dev. 4:1128, 1990; Oro et al., Nature 347:298, 1990). This relationship led to the elucidation of the following properties of Usp: (i) Usp can functionally substitute for RXR in dimerizing with the vertebrate nuclear receptors thyroid hormone receptor (TR), vitamin D receptor (VDR), and peroxisome proliferator-activated receptor (PPAR) (Yao et al., Cell 71:63, 1992); (ii) Usp cannot bind or respond to retinoic acid ligands (Yao, 1992, supra; (iii) Usp heterodimerizes with the ecdysone receptor (EcR) and thereby confers on EcR the ability to mediate ecdysone responses (Oro et al., Develop. 115:449, 1992; Yao et al., Cell 71:63, 1992; Yao et al., Nature 366:476, 1993); (iv) The ability of the EcR/Usp complex to mediate ecdysone responses can be repressed by three other Drosophila hormone orphan receptors (DHRs): DHR78, DHR38, and sevenup (svp) (Zelhof et al., Proc.Natl.Acad.Sci.USA 92:10477, 1995; Zelhof et al., Mol.Cell.Biol. 15:6736, 1995; Sutherland et al., Proc.Natl.Acad.Sci.USA 92:7966, 1995); and (v) DHR38 competes with EcR for heterodimerization with Usp and thereby promotes dissociation of the EcR/Usp heterodimer (Sutherland et al., Proc.Natl.Acad.Sci.USA 92:7966, 1995). These properties indicate that the Usp-RXR system has been evolutionarily conserved; that Usp, similar to RXR, may use heterodimerization as a mechanism to coordinate the function of multiple signal transduction pathways; and that Usp and RXR are functionally distinct.
These properties of Usp implicate it as an important target for insecticides. In particular, the role of Usp in insect development, and the differences between Usp and vertebrate RxR proteins, implicate it as a target for environmentally safe compounds that act as insect growth regulators (IGRs) (Graf, Parasitology Today 9:471, 1993). Thus, there is a need in the art for compositions and methods useful for identifying compounds that selectively interfere with the function of Usp.